This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-402314, filed Dec. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a shadow mask and a color cathode ray tube provided with the shadow mask.
2. Description of the Related Art
In general, color cathode ray tubes that are used in color TV sets, color terminal displays, etc., comprise an envelope that includes a substantially rectangular face panel and a funnel bonded integrally to the face panel. A phosphor screen of the black-matrix or -stripe type having three-color phosphor layers that glow blue, green, and red, individually, is formed on the inner surface of the face panel.
In the envelope, a shadow mask for color sorting is opposed to the phosphor screen. The shadow mask is formed having a large number of electron beam passage apertures through which electron beams pass. The shadow mask is fixed to a mask frame, which is attached to the inner surface of the face panel by means of stud pins. A magnetic shielding plate is also attached to the mask frame.
Located in a neck of the funnel, moreover, is an in-line electron gun, which emits electron beams including a center beams and a pair of side beams. In the color cathode ray tube, the electron beams emitted from the electron gun are deflected by a magnetic field, which is generated by a deflection yoke on the outside of the funnel, and scan the phosphor screen in the horizontal and vertical directions through the shadow mask, thereby displaying a color image on the screen.
The shadow mask is made of a substantially rectangular metal sheet having a large number of electron beam passage apertures formed by etching. The electron beam passage apertures can be roughly classified into two types, a circular-dot type and a rectangular-slit type. Shadow masks having electron beam passage apertures of the circular-dot type are mainly used for display tubes that primarily display characters and graphics. On the other hand, shadow masks having electron beam passage apertures of the rectangular-slit type are mainly used for consumer color cathode ray tubes for color TV sets and the like.
Basically, in the shadow mask of either type, each electron beam passage aperture is formed of a communication hole, in which a larger hole in that surface of the shadow mask which faces the phosphor screen connects with a smaller hole in the opposite surface of the mask that faces the electron gun. The aperture of the electron beam passage aperture is practically settled depending on the diameter of a joint portion at which the respective bottoms of the larger and smaller holes are connected to each other.
In the central portion of an effective area of the shadow mask, the joint portion of each electron beam passage aperture is situated eccentrically on the electron gun side of the thickness-direction center of the shadow mask, while the larger and smaller holes of each electron beam passage aperture are concentric with each other.
In those peripheral portions of the effective area of the shadow mask which are deviated from the region near the minor axis of the effective area in the direction of its major axis, the central axis of the larger hole of each electron beam passage aperture is offset against that of the smaller hole in a direction that recedes from the center of the effective area. Accordingly, the joint portion of each electron beam passage aperture in the peripheral portions is situated nearer to the electron gun side in the thickness-direction of the shadow mask than the joint portion of each electron beam passage aperture in the central portion of the effective area. By use of these offset electron beam passage apertures, the electron beams can be prevented from running against the respective inner surfaces of the electron beam passage apertures or aperture edges of the larger holes in the peripheral portion of the shadow mask where the electron beams are deflected at wider angles. In consequence, the electron beams that are passed through the electron beam passage apertures and landed on the phosphor screen can be prevented from being distorted.
With the development of large-screen versions of color TV sets and the like, on the other hand, flat square tubes have become prevalent consumer color cathode ray tubes. These tubes have a flat screen that reflects less external light and suffers less image distortion. Further, perfectly flat tubes, which have a face panel with a substantially perfectly flat outer surface, have become popular in the market, and are expected to be prevailing color cathode ray tubes for the future.
In one such flat tube, the effective area of the shadow mask is flattened corresponding to the shape of the inner surface of the face panel, so that the shadow mask has a smaller curvature (or a greater radius of curvature) than that of the shadow mask of a conventional color cathode ray tube of which the panel has a curved outer surface.
If the curvature of the shadow mask is reduced in this manner, its curved surface retention (hereinafter referred to as mask strength) lowers, so that it is hard for the mask to maintain its curved surface, resisting its own weight or external force. If the mask strength is low, the curved surface of the shadow mask is inevitably deformed by a minor external force that acts on it during manufacture or transportation. The deformation of the shadow mask changes the distance between the electron beam passage apertures and the inner surface of the panel. In consequence, the electron beams fail to land on the specific phosphor layers, thereby causing a color drift.
If the mask strength is low, moreover, the curved surface of the shadow mask easily resonates with vibration such as a sound from a speaker when the mask is incorporated in a TV set or the like. If the shadow mask resonates, unnecessary light and shade that involve fluctuation in brightness appear on the screen, so that the image quality level lowers.
The mask strength decreases most severely in the central portion of the effective area, and increases as the periphery of the effective area is approached. Thus, if a uniform load is applied to the whole surface of the shadow mask, the mask undergoes a great displacement in the central portion of the effective area and a smaller displacement in the peripheral portion of the effective area. The peripheral portion of the effective area of the shadow mask is provided with a skirt, to which the shadow mask is fixed by welding, so that the mask strength in the peripheral portion of the effective area is increased.
In order to improve the strength of the shadow mask as a whole, therefore, the strength of the central portion of its effective area must be increased. The easiest method to increase the mask strength is to thicken the shadow mask. If the thickness of the shadow mask is increased, however, etching speed control during the manufacture of the mask is so hard that the respective apertures of the electron beam passage apertures are subject to substantial irregularity. In consequence, the yield of production of shadow masks and color cathode ray tubes lowers, and the resulting pictures are subject to unevenness in brightness or color, so that the image quality level is lowered inevitably.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a shadow mask improved in strength without increasing its thickness and a color cathode ray tube capable of ensuring an improved image quality level.
In order to achieve the above object, a shadow mask according to an aspect of the invention comprises a mask body including a substantially rectangular effective area having a minor axis and a major axis extending at right angles to each other and a large number of electron beam passage apertures formed in the effective area. Each of the electron beam passage apertures is formed of a communication hole connecting a larger hole opening in one surface of the effective area and a smaller hole opening in the other surface of the effective area. In a cross section of the mask body in the major axis direction, a joint portion between the larger and smaller holes of each of at least the electron beam passage apertures in the central portion of the effective area is situated in a central portion in the thickness-direction of the mask body. In a cross section of the mask body in the major axis direction, a joint portion between the larger and smaller holes of each of the electron beam passage apertures located on the major axis and in the peripheral portion of the effective area is situated closer to one of the surface sides of the effective area than the joint portion of each of the electron beam passages apertures in the central portion of the effective area, the larger hole being offset against the smaller hole in the direction of the major axis.
A color cathode ray tube according to another aspect of the invention comprises an envelope including a substantially rectangular face panel having a phosphor screen on the inner surface thereof, a shadow mask opposed to the phosphor screen, and an electron gun for emitting electron beams toward the phosphor screen through the shadow mask.
The shadow mask comprises a mask body including a substantially rectangular effective area having a minor axis and a major axis extending at right angles to each other and a large number of electron beam passage apertures formed in the effective area. Each of the electron beam passage apertures is formed of a communication hole connecting a larger hole opening in one surface of the effective area and a smaller hole opening in the other surface of the effective area. In a cross section of the mask body in the major axis direction, a joint portion between the larger and smaller holes of each of at least the electron beam passage apertures in the central portion of the effective area is situated in a central portion in the thickness-direction of the mask body. In a cross section of the mask body in the major axis direction, a joint portion between the larger and smaller holes of each of the electron beam passage apertures located on the major axis and in the peripheral portion of the effective area is situated closer to one of the surface sides of the effective area than the joint portion of each of the electron beam passages apertures in the central portion of the effective area, the larger hole being offset against the smaller hole in the direction of the major axis.
According to the shadow mask and the color cathode ray tube constructed in this manner, the extent of etching of the shadow mask for the formation of the electron beam passage apertures can be lowered despite the equality of the shadow mask to a conventional one in thickness. Thus, the volume of the etched shadow mask can be increased to improve the mask strength.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.